Background and established knowledge Periodontal disease is a family of bacterial infections characterized by the destruction of periodontal supporting tissues. The bacterial flora at the diseased sites is complex, counting > 500 different species in the subgingival dental plaque. Three main hypotheses exist in explaining the infectious nature of periodontal disease; the non-specific infection hypothesis (Theilade 1986), the specific infection hypothesis (Slots and Rams 1991, Loesche 1992) and the ecological plaque hypothesis (Marsh 1994).